German Published Patent Application No. 198 26 047 describes a hydraulic valve actuator of this type, which is also referred to as an “actuator”. In this actuator, the lower pressure chamber, via which the operating piston is displaced in the direction of valve closing, is continually charged with pressurized fluid. The upper pressure chamber, provided with an intake line and a return line, via which a piston displacement in the direction of valve opening is effected, is selectively charged with pressurized fluid via the intake, using control valves, such as 2/2 solenoid valves, or it is relieved again to approximately ambient pressure via the return line. A regulated pressure-supply device supplies the pressurized fluid. Of the control valves, a first control valve connects the upper pressure chamber to a relief line discharging into a fluid reservoir, and a second control valve connects the upper pressure chamber to the pressure-supply device. In the closed state of the gas exchange valve, the upper pressure chamber is disconnected from the pressure supply device by the closed second control valve and connected to the relief line via the open first control valve, so that the actuating piston is retained in its closed position by the fluid pressure prevailing in the lower pressure chamber. To open the gas exchange valve, the control valves are switched over, so that the upper pressure chamber is cut off from the relief line and connected to the pressure supply device. The gas-exchange valve opens because the effective area of the operating piston delimiting the upper pressure chamber is larger than the effective area of the operating piston delimiting the lower pressure chamber, the magnitude of the opening stroke lift being a function of the generation of the electrical control signal applied to the second control valve, and the opening speed being a function of the fluid pressure applied by the pressure-supply device. To close the gas exchange valve, the control valves are switched over again, thereby connecting the upper pressure chamber, which is blocked off from the pressure supply device, to the relief line. The fluid pressure prevailing in the lower pressure chamber guides the operating piston back into its upper limit position, so that the gas exchange valve is closed by the operating piston.
Such a device requires rapid closing of the gas exchange valve and, at the same time, a low impact speed of the valve member of the gas-exchange valve on the valve seat formed in the cylinder head of the combustion cylinder. For reasons of noise and wear, this speed must not exceed certain limit values.
To this end, the use of a valve brake has been proposed in German Published Patent Application No. 102 01 167.2, which brake is connected to the valve member of the gas-exchange valve or to the valve actuator. The valve brake, which acts during a residual closing stroke of the valve member, includes an hydraulic damping member having a fluid displacement volume that discharges via a throttle opening. In one version, where the damping member is integrated in the valve actuator, the return line of the upper pressure chamber is split between two discharge orifices, which are connected to one another and arranged in the housing with axial clearance. A restrictor is assigned to the upper discharge orifice, and the lower discharge orifice is situated in the displacement path of the operating piston in such a way that it may be closed by the operating piston prior to reaching the upper limit position. The throttle opening is realized by a pressure-controlled restrictor whose control pressure is adjusted as a function of the viscosity of the displacement volume with the aid of an electrically controlled hydraulic pressure valve and an electronic control device that triggers it. This has the advantage that the valve member is decelerated during the closing stroke before it reaches its closed position, the braking effect being independent of the temperature and the resulting viscosity of the fluid volume displaced via the throttle opening. Since the opening cross section of the throttle opening is reduced with increasing temperature and attendant decreasing viscosity, the flow velocity of the displaced fluid volume through the throttle opening is reduced to the same extent, so that the magnitude of the braking of the operating piston via the damping member remains approximately constant.